The carbon nanotubes (CNT) possess unique properties due to extremely small nanotube diameter (˜1 nm for a single walled CNT,) which gives rise to a strong two-dimensional quantization of the energy spectrum. As a result, the electron scattering is largely suppressed and the electrons move along the nanotube ballistically, i.e. without collisions and loss of energy, while the nanotube resistance is essentially controlled by the contact resistances. The minimum contact resistance is determined by the quantum contact resistance of 6.5 kOhm (per contact).
In the previous patent U.S. Pat. No. 7,102,157, the ballistic electron propagation along the nanotube was utilized to disclose a vacuum electron emitter, in which electrons under the potential difference between the contacts V approaching the φ/e, where φ is the nanotube work function (˜4.7 eV for CNT), will be able to escape into vacuum and be collected with the external electrode (anode).
It should be emphasized that the physical meaning of the ballistic transport is more stringent than simple preservation of the electron energy, which is needed for the invented devices, according to the present invention. It requires a phase coherent resonance for electron propagation, like a Fabry-Perot resonance for the light propagation in the laser, see e.g. W. Liang et al, Nature, 411,665, 2001; or J. Kong et al, Phys. Rev. Lett. 87,106801, 2001. In the disclosed devices, the elastic electron scattering, such as impurity scattering affecting the electron phase, is not important since it does not change the electron energy. In the description below, the “ballistic” electron transport implies only the energy conservation during the electron movement within the nanotube.
FIG. 1 illustrates as a Prior Art the ballistic mechanism of electron escape into vacuum discussed in the above cited patent. A CNT is placed between two contacts, which form the emitter circuit. FIG. 1a shows the energy band diagram. The contacts are shown as potential barriers in conjunction with electron reservoirs of the metal electrodes, while the CNT is shown as a low electron density semiconductor 90. The applied voltage V, see FIG. 1b, is distributed across the input and output contacts as Vin and Vout respectively, according to the resistance of the barriers. The electrons tunnel through the input barrier into the nanotube and move ballisticaly (i.e. without energy loss) through high energy states to the positively biased output contact. This implies that electrons gain the energy eVin. It is preferable to make the tunneling resistance of the input contact much higher than that of the output contact, i.e. Vin/Vout>>1. Then almost entire applied voltage V will drop across the input contact, V˜Vin. If this energy exceeds the work function φ, the electrons at this contact are ready to escape into vacuum and can be extracted with the anode voltage Va>V. It was assumed however that for high energy electrons there is large probability of electron energy relaxation due to electron-phonon and electron-electron interactions, so that only a fraction of electron will be able to escape into vacuum.
Strongly quantized energy spectrum of the nanotubes due to extremely small nanotube diameter dramatically suppresses the electron-phonon interaction thereby making ballistic electron propagation possible. At present, there is a lot of evidence for the ballistic electron movement in the SWNT, both at low voltages (see e.g. W. Liang, et al, Nature 411, 665, 2001 for metallic nanotubes and A. Javey et al, Nature, 424, 654, 2003 for semiconducting SWNT) and at higher voltages, see e.g. Z. Yao et al, Phys. Rev. Lett. 84, 2941, 2000. In the latter article, the mean free path of the high energy electrons for the backscattering optical phonon emission in SWNT, lo, was estimated to be very large reaching ˜100 nm. This implies that for the nanotube of a length comparable to lo there is a large probability for the electron escape into vacuum.
In the cited above publication by A. Javey et al, a ballistic field-effect transistor was described, wherein the ballistic electron propagation between source and drain contacts on the CWNT was modulated by the gate electrode. The contacts to the nanotube were made with palladium (Pd) which minimizes the contact resistance to its quantum limit of 6.5 KOhm. Other metals form a Schottky barrier with the nanotube, with the tunneling resistance varying in a large range, typically from 10 KOhm to 1 MOhm. In the foregoing publication by A. Javey et al, the field-effect transistor, although also relying on the ballistic electron movement along the nanotube, does not employ electron escape into vacuum, as discussed in the disclosed below ballistic transistors. It should also be emphasized that the ballistic mechanism for electron emission, according to the present invention, is fundamentally different from the field induced electron escape from the nanotube, wherein the tip serves as a concentrator of the electric field, while the nanotube itself represents merely a conductor.
In the cited above U.S. Pat. No. 7,102,157, the ballistic device is made as a single nanotube placed on the insulating substrate and endowed with two contacts at the nanotube ends, see FIG. 2 (Prior Art). In this configuration, parallel placement of multiple identical nanotubes to enhance the output current requires a special manipulator and seems impractical for the device manufacturing. It would be therefore preferable to grow a controllable nanotube array on a metal electrode normally to the electrode plane and then attach a second common contact to all the nanotube tips.
Such a design was partially disclosed by Z. F. Ren et al. in the US patent application # US 20040058153 A1 for fabrication of the field-emission cathode. In this design, the grown nanotubes were covered with an insulating layer, and nanotube ends protrude above this layer. Then the top side is polished to cut off the nanotube ends thus exposing the tips of the nanotubes. The nanotube array in the Ren's patent application was used solely to produce a diode-like cathode-to-anode structure for the field-induced electron emission from the nanotube into vacuum due to electric field focusing at the nanotube tips. Neither second contact to the nanotube tips nor ballistic electron movement along the nanotubes are discussed there.
In the below disclosed designs, according to the present invention, a second contact is placed on the insulator layer covering the nanotubes and is connected to the nanotube tips, thereby forming, together with the bottom electrode and the external anode electrode, a three-terminal hybrid (solid-state/vacuum) device structures suitable for producing an output, anode, current due to the ballistic electron emission from the nanotube. All the nanotube tips covered by the top contact equally participate in the emission process thereby multiplying the output current.
Several different ballistic device embodiments are disclosed below. In all of them, the multilayer device structure with the flat platform for the second nanotube contact allow fabrication of the nanotube arrays with extremely short nanotube length, which is necessary for obtaining a sizable ballistic electron emission effect.
The last disclosed nanotube array electronic device is based on similar technique for making a flat platform at the nanotube tip but has non-ballistic nature for the electron emission. This is the only disclosed device which relies on the field-induced electron emission from the nanotube tips.